For me it is the fact that our blood contains iron. I earlier used to believe the word stood for some ‘organic element’ since I couldn’t accept we had metal flowing through our supposed carbon-based bodies, till I realized that is where the taste and smell of blood comes from.
But it does. If the universe was deterministic, choice would be impossible because all outcomes would be predetermined.
Quantum randomness may not directly provide free will but it does exclude determinism, which would make free will impossible.